Photomask blank and photomask

ABSTRACT

A photomask blank to be used as a material for a photomask is provided with a mask pattern having a transparent area and an effectively opaque area to exposure light on a transparent substrate. On the transparent board, one or more layers of light shielding films are formed with or without other film (A) in between, at least one layer (B) which constitutes the light shielding film includes silicon and a transition metal as main component, and a molar ratio of silicon to the transition metal is silicon:metal=4-15:1 (atomic ratio). The photomask provided with the mask pattern having the transparent area and the effectively opaque area to exposure light on the transparent board is also provided.

CROSS-REFERENCE

This application is a Divisional of co-pending U.S. application Ser. No.11/662,183, filed on Mar. 8, 2007, which is the national phase ofPCT/JP2005/016511 filed on Sep. 8, 2005, which designated the UnitedStates and which claims priority to Japanese Application 2004-263161filed on Sep. 10, 2004. The entire contents of the above applicationsare hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to photomask blanks and photomasks for use in themicrofabrication of semiconductor integrated circuits, charge coupleddevices (CCD), liquid crystal display (LCD) color filters, magneticheads or the like.

BACKGROUND ART

In the recent semiconductor processing technology, a challenge to higherintegration of large-scale integrated circuits places an increasingdemand for miniaturization of circuit patterns. There are increasingdemands for further reduction in size of circuit-constructing wiringpatterns and for miniaturization of contact hole patterns forcell-constructing inter-layer connections. As a consequence, in themanufacture of circuit pattern-written photomasks for use in thephotolithography of forming such wiring patterns and contact holepatterns, a technique capable of accurately writing finer circuitpatterns is needed to meet the miniaturization demand.

In order to form a higher accuracy photomask pattern on a photomasksubstrate, it is of first priority to form a high accuracy resistpattern on a photomask blank. Since the photolithography carries outreduction projection in actually processing semiconductor substrates,the photomask pattern has a size of about 4 times the actually necessarypattern size, but an accuracy which is not loosened accordingly. Thephotomask serving as an original is rather required to have an accuracywhich is higher than the pattern accuracy following exposure.

Further, in the currently prevailing lithography, a circuit pattern tobe written has a size far smaller than the wavelength of light exposed.If a photomask pattern which is a mere 4-time magnification of thecircuit feature is used, a shape corresponding to the photomask patternis not transferred to the resist film due to influences such as opticalinterference occurring in the actual photolithography operation. Tomitigate these influences, in some cases, the photomask pattern must bedesigned to a shape which is more complex than the actual circuitpattern, i.e., a shape to which the so-called optical and proximitycorrection (OPC) is applied. Then, at the present, the lithographytechnology for obtaining photomask patterns also requires a higheraccuracy processing method. The lithographic performance is sometimesrepresented by a maximum resolution. As to the resolution limit, thelithography involved in the photomask processing step is required tohave a maximum resolution accuracy which is equal to or greater than theresolution limit necessary for the photolithography used in asemiconductor processing step using a photomask.

A photomask pattern is generally formed by forming a photoresist film ona photomask blank having a light-shielding film on a transparentsubstrate, writing a pattern using electron beam, and developing to forma resist pattern. Using the resulting resist pattern as an etch mask,the light-shielding film is etched into a light-shielding pattern. In anattempt to miniaturize the light-shielding pattern, if processing iscarried out while maintaining the thickness of the resist film at thesame level as in the prior art prior to the miniaturization, the ratioof film thickness to pattern, known as aspect ratio, becomes higher. Asa result, the resist pattern profile is degraded, preventing effectivepattern transfer, and in some cases, there occurs resist patterncollapse or stripping. Therefore, the miniaturization must entail athickness reduction of resist film.

As to the light-shielding film material which is etched using the resistas an etch mask, on the other hand, a number of materials have beenproposed. In practice, chromium compound films are always employedbecause there are known a number of studies with respect to theiretching and the standard process has been established. Typical of suchfilms are light-shielding films composed of chromium compounds necessaryfor photomask blanks for ArF excimer laser lithography, which includechromium compound films with a thickness of 50 to 77 nm as reported inJP-A 2003-195479 (Patent Reference 1), JP-A 2003-195483 (PatentReference 2), and Japanese Patent No. 3093632 (Patent Reference 3).

However, oxygen-containing chlorine dry etching which is a common dryetching process for chromium based films such as chromium compound filmsoften has a capability of etching organic films to some extent. Ifetching is carried out through a thin resist film, accurate transfer ofthe resist pattern is difficult. It is a task of some difficulty for theresist to have both a high resolution and etch resistance that allowsfor high accuracy etching. Then, for the purpose of achieving highresolution and high accuracy, the light-shielding film material has tobe reviewed so as to find a transition from the approach relying only onthe resist performance to the approach of improving the light-shieldingfilm performance as well.

Also, as to light-shielding film materials other than the chromium basedmaterials, a number of studies have been made. One example of the lateststudies is the use of tantalum in the light-shielding film for ArFexcimer laser lithography. See JP-A 2001-312043 (Patent Reference 4).

On the other hand, it has long been a common practice to use a hard maskfor reducing the load on resist during dry etching. For example, JP-A63-85553 (Patent Reference 5) discloses MoSi₂ overlaid with a SiO₂ film,which is used as an etch mask during dry etching of MoSi₂. It isdescribed that the SiO₂ film can also function as an antireflectivefilm.

From the past, studies have been made on metal silicide films,especially molybdenum silicide films, which can be readily etched underetching conditions for fluorine dry etching that causes few damages toresist film. They are disclosed, for example, in JP-A 63-85553 (PatentReference 5), JP-A 1-142637 (Patent Reference 6), and JP-A 3-116147(Patent Reference 7), all of which basically use a film of silicon andmolybdenum=2:1. Also, JP-A 4-246649 (Patent Reference 8) discloses ametal silicide film. All these metal silicide films have insufficientchemical stability to chemical cleaning as the final step of thephotomask fabrication process, and particularly when the films are madethin, may lose the physical properties the film should maintain duringthe cleaning step.

Patent Reference 1: JP-A 2003-195479

Patent Reference 2: JP-A 2003-195483

Patent Reference 3: Japanese Patent 3093632

Patent Reference 4: JP-A 2001-312043

Patent Reference 5: JP-A 63-85553

Patent Reference 6: JP-A 1-142637

Patent Reference 7: JP-A 3-116147

Patent Reference 8: JP-A 4-246649

Patent Reference 9: JP-A 7-140635

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

The present invention has been made in order to solve the aboveproblems, and its object is to provide a photomask blank which endows aphotomask with both a high resolution and a high accuracy etchingcapability for forming a finer photomask pattern, especially as neededin the photolithography involving exposure to light of a wavelengthequal to or less than 250 nm such as ArF excimer laser light, i.e., aphotomask blank comprising a light-shielding film which can be etchedwithout any substantial load to the resist during etching operation andwhich has sufficient chemical stability during the mask cleaning steprequisite in the photomask manufacture process; and a photomask having amask pattern formed using the photomask blank.

Means for Solving the Problem

Making extensive investigations to solve the outstanding problems, theinventor has found that a film containing silicon and a transition metalin a specific ratio has higher light-shielding property to exposurelight of a wavelength equal to or less than 250 nm, especially ArFexcimer laser light, than the chromium based films which are used in theart, and that although the chemical stability is believed low in theart, it is chemically stable when the ratio falls in the specific range.The invention is predicated on this finding.

Accordingly, the present invention provides a photomask blank and aphotomask as defined below.

Item 1:

A photomask blank from which is produced a photomask comprising atransparent substrate and a mask pattern formed thereon includingtransparent regions and effectively opaque regions to exposure light,characterized in that a light-shielding film of one or more layers isformed on a transparent substrate with or without another film (A)intervening therebetween, at least one layer (B) of the layers of whichthe light-shielding film is composed contains silicon and a transitionmetal as main components, and the silicon and the transition metal arepresent at a silicon/metal molar ratio of 4-15:1 (atomic ratio).

Item 2:

The photomask blank of item 1, wherein the transition metal ismolybdenum.

Item 3:

The photomask blank of items 1 or 2, wherein the at least one layer (B)of the layers of which the light shielding film is composed furthercontains at least one element selected from oxygen, nitrogen and carbon.

Item 4:

The photomask blank of any one of items 1 to 3, wherein saidlight-shielding film has a thickness of 20 to 50 nm.

Item 5:

The photomask blank of any one of items 1 to 4, wherein saidlight-shielding film is overlaid with an antireflective film, and theantireflective film contains as a main component a transition metalsilicide oxide, transition metal silicide nitride, transition metalsilicide oxynitride, transition metal silicide oxycarbide, transitionmetal silicide carbonitride or transition metal silicide oxide nitridecarbide.

Item 6:

The photomask blank of item 5, wherein the transition metal silicide ismolybdenum silicide.

Item 7:

The photomask blank of any one of items 1 to 4, wherein saidlight-shielding film is overlaid with an antireflective film, and theantireflective film contains as a main component a chromium oxide,chromium nitride, chromium oxynitride, chromium oxycarbide, chromiumcarbonitride or chromium oxide nitride carbide.

Item 8:

The photomask blank of any one of items 1 to 7, wherein the other film(A) comprises a phase shift film.

Item 9:

A photomask comprising a transparent substrate and a mask pattern formedthereon including transparent regions and effectively opaque regions toexposure light, which is produced from the photomask blank of any one ofitems 1 to 8.

In the invention, an antireflective film may be disposed on thelight-shielding film. In one embodiment wherein the antireflective filmis made of a transition metal silicide compound such as a transitionmetal silicide oxide, transition metal silicide nitride, transitionmetal silicide oxynitride, transition metal silicide oxycarbide,transition metal silicide carbonitride or transition metal silicideoxide nitride carbide, the light-shielding film and the antireflectivefilm can be etched by fluorine dry etching, ensuring very high etchingprocessability.

In another embodiment wherein the antireflective film is made of achromium compound such as a chromium oxide, chromium nitride, chromiumoxynitride, chromium oxycarbide, chromium carbonitride or chromium oxidenitride carbide, the light-shielding film and the antireflective filmare such that they can be fully processed using a thin resist film. Theycan be processed without substantial damage to the resist, and thechemical stability during cleaning is satisfactory.

BENEFITS OF THE INVENTION

The photomask blank having a light-shielding film as constructedaccording to the invention is a photomask blank having a light-shieldingfilm possessing high light-shielding property and chemical stability,which can be etched or processed under sufficient etching conditions orwithin a sufficient etching time to minimize the damage to the resistduring etching, even when the light-shielding film is overlaid with anantireflective film. Then the resist can be formed relatively thin,thereby avoiding the problems associated with an increase of resist filmaspect ratio and enabling higher accuracy photomask pattern formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a DC sputtering apparatus having twotargets used in Examples.

FIG. 2 is a graph showing the film thickness dependency of the opticaldensity of a light-shielding film of Example 1 relative to light ofwavelength 248 nm and 193 nm.

FIG. 3 is a graph showing the film thickness dependency of the opticaldensity of light-shielding films of Example 1 and Comparative Examples 1and 2 relative to light of wavelength 193 nm.

FIG. 4 is a graph showing the film thickness dependency of the opticaldensity of a chromium film relative to light of wavelength 193 nm.

FIG. 5 is a graph showing the wavelength dependency of the opticaldensity of a light-shielding film of Example 1.

FIG. 6 is a graph showing the wavelength dependency of the reflectanceof antireflective films on photomask blanks of Examples 6-9.

BEST MODE FOR CARRYING OUT THE INVENTION

Now the invention is described in further detail.

From the photomask blank of the invention is produced a photomaskcomprising a transparent substrate and a mask pattern formed thereonincluding transparent regions which are transparent to exposure lightand regions which are effectively opaque to exposure light, that is,sufficiently opaque to provide a practical light shield when used inpattern exposure as a photomask. The photomask blank is constructed byforming a light-shielding film of one or more layers on a transparentsubstrate with or without another film intervening therebetween. Atleast one layer of the layers of which the light-shielding film iscomposed contains silicon and a transition metal as main components, andthe silicon and the transition metal are present at a silicon/metalmolar ratio of 4-15:1 (atomic ratio).

In the photomask blank of the invention, the light-shielding film may beeither a single layer film or a multilayer film. At least one layer ofthe layers of which the light-shielding film is composed should containsilicon and a transition metal, wherein the silicon and the transitionmetal are present at a silicon/metal molar ratio of 4-15:1 (atomicratio), that is, between 4:1 and 15:1 (atomic ratio). In particular, asingle layer film is preferred in order to provide higherprocessability. For the multilayer film, it is preferred that all thelayers of which the light-shielding film is composed contain silicon anda transition metal as main components wherein the silicon and thetransition metal are present at a silicon/metal molar ratio of 4-15:1(atomic ratio). It is noted for the multilayer film that a tungstenlayer, tantalum layer or the like may be formed as a layer other thanthe silicon and transition metal-containing layer, specifically betweenthe silicon and transition metal-containing layer and the transparentsubstrate.

The light-shielding film must have such chemical stability that itundergoes no thickness changes during cleaning. For the ArF lithographyphotomask application, a film thickness change equal to or less than 3nm during cleaning is required. It is to be noted that thelight-shielding film can be damaged under conditions of cleaningrequisite to the photomask manufacture process, especially cleaning witha sulfuric acid-hydrogen peroxide mixture (SPM) so that thelight-shielding performance is lost. Also attention should be paid tothe electric conductivity of the film in order to avoid the film fromcharging-up upon exposure to electron beam in the lithography step formask pattern formation. The light-shielding film of the inventionwherein the molar ratio of silicon to transition metal is within theabove-specified range ensures that a light-shielding film has a chemicalstability and electric conductivity falling within the ranges ofpractically acceptable physical properties.

Examples of suitable transition metals of which the light-shielding filmis composed include molybdenum, tantalum, tungsten, cobalt, nickel,vanadium, titanium, niobium, zirconium, hafnium, and the like.Molybdenum is most preferred for dry etching processability.

In order that a film deposited on a photomask function as a film havingsufficient light-shielding ability, in the case of a commonly usedbinary mask blank comprising a light-shielding film and anantireflective film, the combination of light-shielding film andantireflective film, and in the case of a halftone phase shift maskblank, the combination of a halftone phase shift film, light-shieldingfilm and antireflective film should have an optical density OD of atleast 2.5, specifically at least 2.8, more specifically at least 3.0relative to exposure light. Then, the film containing silicon andtransition metal as main components may consist essentially of siliconand transition metal, or may further contain light elements such asoxygen, nitrogen, carbon or the like as additional components. Sincesufficient light-shielding is not available sometimes when these lightelements are contained above a certain level, it is desired for apreferred embodiment of the inventive photomask blank, i.e., a photomaskblank adapted for ArF excimer laser lithography at wavelength 193 nmthat the nitrogen and carbon contents each be equal to or less than 20atom % and the oxygen content be equal to or less than 10 atom %, andespecially the total content of nitrogen, carbon, and oxygen be equal toor less than 40 atom %.

Also, the light-shielding film preferably has a thickness of 20 to 50nm. At a film thickness less than 20 nm, sufficient light-shieldingeffect is not available in some cases. In excess of 50 nm, high accuracyprocessing with a thin resist having a thickness equal to or less than250 nm may become difficult, or film stress may cause warpage to thesubstrate.

The light-shielding film can be formed by well-known methods. Filmformation by sputtering is commonly used as the simplest method offorming a homogeneous film, and the sputtering is the preferred filmformation method in the present invention as well. The target used maybe a single target containing silicon and transition metal in acontrolled ratio from 4:1 to 15:1. Alternatively, a ratio of silicon totransition metal may be adjusted by selecting appropriate ones from asilicon target, a transition metal target, and targets of silicon andtransition metal (transition metal silicide targets) and controlling thesputtering area of the selected targets or the power applied to theselected targets. It is noted that when the light-shielding filmcontains light elements such as oxygen, nitrogen, and carbon, such afilm can be deposited by reactive sputtering wherein anoxygen-containing gas, nitrogen-containing gas or carbon-containing gasis added to the sputtering gas as a reactive gas.

In the practice of the invention, the light-shielding film describedabove may be overlaid with an antireflective film Generally theantireflective film used herein may be any of well-known films althoughthe following two embodiments are often used for processability.

One embodiment is an antireflective film which is suitable when theantireflective film and the light-shielding film are simultaneouslyetched using the resist as an etch mask. This antireflective filmcontains as a main component a transition metal silicide compound suchas a transition metal silicide oxide, transition metal silicide nitride,transition metal silicide oxynitride, transition metal silicideoxycarbide, transition metal silicide carbonitride, transition metalsilicide oxide nitride carbide and the like. As the transition metalused herein, the exemplary transition metals illustrated for thelight-shielding film are preferred again. For etching processability,the transition metal used herein is preferably the same as in thelight-shielding film, with molybdenum being most preferred.

In this embodiment, the antireflective film preferably has an atomiccomposition which is set to the range of 0.2 to 25 atom % transitionmetal, 10 to 57 atom % Si, 0 to 60 atom % O, 0 to 57 atom % N, and 0 to30 atom % C, and to provide an optical density OD between 0.3 and 1.5,preferably between 0.5 and 1.0, relative to exposure light when itsthickness is within the range described below. Although the thickness ofthe antireflective film varies with the wavelength of light used forinspection necessary during fabrication or use of the photomask, theantireflection effect is generally exerted at a thickness of 15 to 30nm. A thickness of 20 to 25 nm is preferred especially for the ArFexcimer laser lithography. Since the antireflective film has dry etchingcharacteristics equivalent to those of the light-shielding film, thelight-shielding film and the antireflective film can be etched in asingle step to form a light-shielding pattern.

The antireflective film can be formed by well-known methods. The methodoften used herein is by selecting an appropriate target or targets froma silicon target, transition metal target, and silicon and transitionmetal-containing targets (metal silicide targets), and performingreactive sputtering in a reactive gas or a gas mixture of a reactive gasand an inert gas such as argon. See JP-A 7-140635 (Patent Reference 9).

The other embodiment is an antireflective film containing as a maincomponent a chromium compound such as a chromium oxide, chromiumnitride, chromium oxynitride, chromium oxycarbide, chromiumcarbonitride, chromium oxide nitride carbide or the like. The chlorinedry etching which is a typical etching process for chromium compoundscan cause damages to the resist as previously pointed out. If the filmis used solely as an antireflective film, a film thickness of about 15to 30 nm is sufficient, in which thickness range an antireflective filmof chromium compound can be etched to completion before causing anysignificant damage to the resist.

The etching of this antireflective film, that is, chlorine dry etchingcannot etch the light-shielding film of the invention. Once theantireflective film of chromium compound is etched, a light-shieldingfilm containing silicon and transition metal is etched by fluorine dryetching using the antireflective film as an etch mask. Then due to thehigh etch resistance of chromium compound, the antireflective filmfunctions as an etch mask so that high accuracy etching is expectable.This embodiment is preferred in the case of deep trenching at theetching stage, for example, when the mask is used as Levenson-type phaseshift mask.

In this embodiment, the antireflective film preferably has an atomiccomposition which is set to the range of 30 to 85 atom % Cr, 0 to 60atom % O, 0 to 50 atom % N, and 0 to 20 atom % C, and to provide anoptical density OD between 0.3 and 1.5, preferably between 0.5 and 1.0,relative to exposure light when its thickness is within the rangedescribed below. Although the thickness of the antireflective filmvaries with the wavelength of light used for inspection necessary duringfabrication or use of the photomask, the antireflection effect isgenerally exerted at a thickness of 15 to 30 nm. A thickness of 20 to 25nm is preferred especially for the ArF lithography.

The antireflective film of this type can be formed by well-knownmethods. The method often used herein is by using a chromium target, andperforming reactive sputtering in a reactive gas or a gas mixture of areactive gas and an inert gas such as argon. See JP-A 7-140635 (PatentReference 9).

In the photomask blank of the invention, another film different from thelight-shielding film and the antireflective film may be provided betweenthe transparent substrate and the light-shielding film, for example, anetch stop film, a translucent film, a phase shift film of MoSi orMoZrSi, or the like.

Described below is the method of producing a photomask using thephotomask blank of the invention. As described above, the process ofprocessing the photomask blank of the invention traverses some differentsteps depending on whether a transition metal silicide compound or achromium compound is used for the antireflective film. Reference isfirst made to the embodiment wherein the antireflective film is atransition metal silicide compound.

First, a resist pattern for writing a circuitry image is formed on thephotomask blank having an antireflective film of transition metalsilicide compound. In this step, surface treatment is preferably carriedout for reducing the surface energy of the substrate (photomask blank)surface, prior to coating of the resist. The best mode of surfacetreatment is by alkylsilylating the surface with hexamethylenedisilazane (HMDS) or other organosilicon surface treating agentscommonly used in the semiconductor manufacture process, which ispreferably implemented by exposing the substrate to the treating agentgas or by directly coating the treating agent to the surface. Thesurface treatment minimizes the occurrence of such problems as finepattern collapse and stripping.

Next, a resist material is coated on the surface treated substrate(photomask blank) and dried to form a resist film. An appropriate resistmust be selected in accordance with the image writing system used.Preferably, positive or negative resist materials comprising aromaticskeleton-bearing polymers are used for the commonly used EB writingprocess, and chemically amplified resist materials are used for themicropatterning photomask manufacture process where the invention iseffectively applicable.

The resist film should have a thickness within the range where it canform a satisfactory pattern profile and function as an etch mask.Particularly when it is desired to form a fine pattern as an ArFlithography mask, the film thickness is preferably equal to or less than350 nm, and more preferably equal to or less than 250 nm. Generally thelower limit of the resist film thickness is preferably equal to orgreater than 75 nm, and more preferably equal to or greater than 100 nm,although it depends on the etch resistance of the resist. It is notedthat the film thickness may be further reduced where a bilayer resistprocess using a silicone resin based resist in combination with anaromatic resin based bottom layer film or a surface imaging processusing an aromatic chemically amplified resist in combination with asilicone base surface treating agent is utilized. Coating conditions anddrying means are selected appropriate for a particular resist used.

Image writing on a resist may be performed by EB irradiation or lightirradiation. In general, EB irradiation is a preferred method forforming fine patterns. Where a chemically amplified resist is used,image writing is generally performed with an energy amount in the rangeof 3 to 30 mC/cm², followed by heat treatment and subsequent developmentto form a resist pattern.

Next, using the resist pattern resulting from the above step as an etchmask, the light-shielding film is etched. In this embodiment of theantireflective film, if the etching is carried out by a well-knownfluorine dry etching process, the antireflective film and thelight-shielding film can be etched simultaneously. It is also possiblethat once the antireflective film is etched, the light-shielding film isetched by a chlorine dry etching process. In this case, a film ofoxygen-rich transition metal silicide compound is not etched, but a filmof oxygen-short transition metal silicide compound is etched. If theoxygen content of the antireflective film is set higher than the oxygencontent of the light-shielding film, then the antireflective film canserve as an etch mask, allowing for higher accuracy processing.

After the light-shielding pattern is formed by etching, the resist isstripped off with a predetermined stripper solution, resulting in aphotomask having the light-shielding film pattern formed thereon. It isnoted that in the case of halftone phase shift masks or Levenson-typephase shift masks, a silicon oxide film which is a typical phase shiftmaterial or a metal silicide compound film such as metal silicideoxynitride film which is a translucent film can be simultaneously etchedunder the same etching conditions as is the light-shielding film. Thenthe invention is advantageously applicable to halftone phase shift masksand Levenson-type phase shift masks.

In the case of phase shift masks, for example, it is a common practicethat a phase shift pattern is formed, after which the light-shieldingfilm pattern is partially removed. In this case, a resist is coatedagain and patterned according to a standard technique, after which theantireflective film and the light-shielding film are etched away by afluorine dry etching process. In this etching, the completion of etchingof the light-shielding film can be judged by a well-known technique, forexample, by detection of etched atoms or detection of reflectance. Also,when the light-shielding film is etched away by a chlorine dry etchingprocess at the end of etching of the antireflective film as mentionedabove, over-etching can be prevented. It is noted that a photomask blankof the construction free of an antireflective film can be processed bythe same process as described above.

Reference is now made to the second embodiment wherein theantireflective film is a chromium compound. It is described how toproduce a photomask by processing the photomask blank. The proceduretaken in this embodiment until a resist pattern is obtained is the sameas in the previous embodiment, i.e., a resist pattern can be formed bythe same procedure as in the embodiment wherein the antireflective filmis a transition metal silicide compound. The next stage is a dry etchingstep where etching is performed by a chlorine dry etching process,especially a chlorine dry etching process using oxygen-laden chlorine.During the chlorine dry etching, an organic film such as a resist can beetched as well, as opposed to fluorine dry etching. However, theantireflective film generally functions at a thickness of about 15 toabout 30 nm, and this etching is completed in a brief time because it isintended only for the antireflective film. Then a resist film having athickness of 100 to 250 nm can be processed at a high accuracy.

For processing at a higher accuracy, the etching of the light-shieldingfilm is changed to fluorine-based dry etching. This type of etching ofthe light-shielding film ensures ease of high-accuracy processing wherethe chromium compound film as the antireflective film plays the role ofan etch mask because the chromium compound film is not etched at all byfluorine-based dry etching. After this step, the resist is stripped by apredetermined technique, completing a photomask. Because the chromiumcompound based antireflective film can be used as an etch mask inprocessing into a Levenson-type phase shift mask or in processing into aphase shift mask having a phase shift film, for example, this procedureis advantageous when the undercoat film between the transparentsubstrate and the light-shielding film must be trenched deeply andaccurately.

The photomask resulting from the above step is finally cleaned withsulfuric acid-hydrogen peroxide mixture and/or aqueous ammonia-hydrogenperoxide mixture, completing the photomask. If the light-shielding filmdoes not have sufficient chemical stability to withstand cleaningconditions during the cleaning, the light-shielding film is over-removedor undercut beneath the antireflective film. This gives rise to theproblem of a significant decline of mask accuracy for binary masks ortri-tone masks using high-transmittance halftone mask blanks, and alsothe problem of a decline of antireflection function where thelight-shielding film is present only at the periphery as in the case oflow-transmittance halftone masks.

In general, in chemical resistance tests of immersing for one hour in anammonia-hydrogen peroxide mixture (aqueous ammonia:hydrogenperoxide:water=1:1:30 in volume ratio) and a sulfuric acid-hydrogenperoxide mixture (sulfuric acid:hydrogen peroxide=4:1 in volume ratio),those films that undergo a thickness change equal to or less than 5 nm,especially equal to or less than 3 nm in both the conditions areregarded satisfactory or free from the above problems. The films of theinvention undergo thickness changes within the range in these chemicalresistance tests, proving excellent chemical stability (or chemicalresistance).

EXAMPLE

Examples and Comparative Examples are given below for illustrating theinvention although the invention is not limited thereto.

Examples 1-5 & Comparative Examples 1-3

Photomask blanks having a light-shielding film deposited on a substratewere prepared by the following procedure.

Using a DC sputtering apparatus having two targets as shown in FIG. 1, alight-shielding film composed of silicon and molybdenum was deposited ona quartz substrate. Illustrated in FIG. 1 are a substrate 1, a chamber101, targets 102 a, 102 b, a sputtering gas inlet 103, an exhaust port104, a substrate rotating platform 105, and power supplies 106 a, 106 b.

Sputtering gases were introduced at the predetermined flow rates shownin Table 1 so as to establish a gas pressure of 0.05 Pa in thesputtering chamber. Two targets were used herein: a Mo target as thetransition metal source and a Si (single crystal) target as the siliconsource. The predetermined discharge powers shown in Table 1 were fed tothe respective targets while the substrate was rotated at 30 rpm. Inthis way, a MoSi film or MoSiON film was deposited to the contents ofsilicon and molybdenum shown in Table 1 and to the predeterminedthickness by controlling the deposition time. The contents of lightelements in the resultant light-shielding film (as determined by ESCA)are also shown in Table 1.

TABLE 1 Sputtering Target gas Light element input power Si:Mo flow ratecontent (W) (atomic (sccm) (atom %) Si Mo ratio) Ar N₂ O₂ NitrogenOxygen Example 1 920 80 9:1 20 0 0 0 0 Example 2 840 160 4:1 20 0 0 0 0Example 3 940 60 15:1  20 0 0 0 0 Example 4 920 80 9:1 20 5 2 11 7Example 5 920 80 9:1 20 10 2 23 6 Comparative 550 450 1:1 20 0 0 0 0Example 1 Comparative 720 280 2:1 20 0 0 0 0 Example 2 Comparative 10000 1:0 20 0 0 0 0 Example 3

Chemical Stability (Chemical Resistance)

Samples having a light-shielding film deposited to a thickness of 39 nmwere immersed for one hour in an ammonia-hydrogen peroxide mixture(aqueous ammonia:hydrogen peroxide:water=1:1:30 in volume ratio) or asulfuric acid-hydrogen peroxide mixture (sulfuric acid:hydrogenperoxide=4:1 in volume ratio), following which a change of filmthickness was determined. The results are shown in Table 2.

Electric Conductivity

For samples having a light-shielding film deposited to a thickness of 39nm, the electric conductivity of the light-shielding film was measuredby a four-probe sheet resistance meter MCP-T600 by Mitsubishi ChemicalCo., Ltd. The results are shown in Table 2.

TABLE 2 Film thickness increase Film thickness reduction by sulfuric byammonia - hydrogen acid - hydrogen peroxide peroxide immersion immersionConductivity (nm) (nm) (Ω/□) Example 1 0.7 0.1 486 Example 2 1.5 0.4 296Example 3 0.2 0 680 Example 4 0.8 0 470 Example 5 0.5 0 584 Comparativefilm loss 26.5 38 Example 1 Comparative 17.7  2.4 96 Example 2Comparative 0.2 0.2 1082 Example 3

Dependency of Optical Density on Film Thickness and Wavelength

For samples in which a light-shielding film was deposited to the varyingthickness shown in Table 3 under the above-described conditions, theoptical density of the light-shielding film was measured by aspectrophotometer, provided that light was incident on the transparentsubstrate side. The results are shown in Table 3. FIG. 2 plots the filmthickness dependency of the optical density of the light-shielding filmof Example 1 relative to light of wavelength 248 nm or 193 nm. FIG. 3plots the film thickness dependency of the optical density of thelight-shielding films of Example 1 and Comparative Examples 1 and 2relative to light of wavelength 193 nm. For reference's sake, theoptical density of metallic Cr film and the film thickness dependencythereof are shown in Table 3 and FIG. 4, respectively.

TABLE 3 OD at 193 nm Example 1 Thickness Thickness Thickness Thickness21 nm 31 nm 39 nm 50 nm 1.56 2.38 3.05 4.00 Example 2 Thickness 39 nm3.02 Example 3 Thickness 38 nm 3.08 Example 4 Thickness 40 nm 2.93Example 5 Thickness 40 nm 2.88 Comparative Example 1 Thickness ThicknessThickness Thickness 39 nm 46 nm 48 nm 60 nm 2.45 3.00 3.15 4.00Comparative Example 2 Thickness Thickness Thickness Thickness 23 nm 33nm 39 nm 55 nm 1.74 2.60 3.05 4.00 Comparative Example 3 Thickness 39 nm3.10 Cr film Thickness Thickness Thickness Thickness 14 nm 24 nm 32 nm46 nm 0.73 1.23 2.08 2.76

The light-shielding film of Example 1 with a thickness of approximately40 nm has an optical density of about 3.0 both at the wavelength of 193nm and 248 nm as seen from FIG. 2, demonstrating superior light shieldto the chromium based light-shielding film (see FIG. 4). By contrast,the light-shielding film of Comparative Example 1 with a thickness ofapproximately 40 nm has an optical density of about 2.5 at thewavelength of 193 nm as seen from FIG. 3, demonstrating no significantdifference in light shield from the metallic chromium light-shieldingfilm (see FIG. 4). The light-shielding film of Comparative Example 3 hasa high sheet resistance, failing to meet electric conductivity.

Further, for the light-shielding film (39 nm thick) of Example 1, thewavelength dependency of its optical density was determined by aspectrophotometer. The results are shown in FIG. 5. As seen from FIG. 5,this light-shielding film has an excellent optical density on the shortwavelength side, demonstrating light-shielding properties suited for DUVlithography.

It is seen from these results that in the photomask blank of theinvention, its light-shielding film has an optical density of around 3at a thickness of about 40 nm relative to light with a wavelength equalto or less than 248 nm and that the photomask blank of the invention anda photomask obtained therefrom have excellent light-shielding propertiesas compared with those using a chromium based film as thelight-shielding film. This suggests that the light-shielding film can bemade thinner, the dry etching time can be reduced accordingly, and animprovement in patterning accuracy due to a thickness reduction of theresist film becomes possible. Furthermore, since the light-shieldingfilm is fully resistant to ammonia-hydrogen peroxide mixtures andsulfuric acid-hydrogen peroxide mixtures commonly used as cleaningsolution in the mask manufacture process, the photomask blank of theinvention undergoes minimized pattern size variations even when cleaningsteps are repeated.

Examples 6-12

Photomask blanks (binary mask blanks) having a light-shielding film andan antireflective film deposited on a substrate were prepared by thefollowing procedure.

Antireflective Film of Molybdenum Silicide Compound Examples 6-9

First, a light-shielding film (Si:Mo=9:1 in atomic ratio) having athickness of 25 nm was deposited under the same conditions as in Example1.

Next, using a DC sputtering apparatus having two targets as shown inFIG. 1, an antireflective film of molybdenum silicide nitride wasdeposited on the light-shielding film. Sputtering gases, Ar gas at aflow rate of 5 sccm, N₂ gas at 50 sccm, and O₂ gas at 0.2 sccm wereintroduced so as to establish a gas pressure of 0.1 Pa in the sputteringchamber. Two targets were used herein: a Mo target as the transitionmetal source and a Si (single crystal) target as the silicon source.Discharge powers of 150 W and 850 W were fed to the Mo and Si targets,respectively, while the substrate was rotated at 30 rpm. In this way, aMoSiN film was deposited so as to contain silicon and molybdenum in aSi:Mo ratio of 4.5:1 (atomic ratio) and to the thickness shown in Table4 by controlling the deposition time.

Antireflective Film of Chromium Compound Examples 10-12

First, a light-shielding film (Si:Mo=9:1 in atomic ratio) having athickness of 39 nm was deposited under the same conditions as in Example1.

Next, using a standard magnetron DC sputtering apparatus having a singletarget, an antireflective film of chromium oxynitride was deposited onthe light-shielding film. Sputtering gases, Ar gas at a flow rate of 10sccm, N₂ gas at 30 sccm, and O₂ gas at 15 sccm were introduced so as toestablish a gas pressure of 0.1 Pa in the sputtering chamber. The targetused herein was a Cr target. A discharge power of 1,000 W was fed to theCr target while the substrate was rotated at 30 rpm. In this way, a CrONfilm was deposited to the thickness shown in Table 4 by controlling thedeposition time.

TABLE 4 Light- shielding Thickness Thickness film (nm) Antireflectivefilm (nm) Example 6 MoSi film 25 MoSiN film 19 Example 7 Si:Mo = 9:1Si:Mo = 4.5:1 23 Example 8 28 Example 9 37 Example 10 39 CrON film 15Example 11 20 Example 12 30

Optical Density

For the above-prepared photomask blanks, the optical density (OD) of thelight-shielding film was measured by a spectrophotometer, provided thatlight was incident on the transparent substrate side. The results areshown in Table 5. An optical density of around 3.0 relative to light ofwavelength 193 nm is obtained when the antireflective film has athickness of 23 nm (summing to 48 nm, combined with the light-shieldingfilm). This, in view of the fact that a chromium based light-shieldingfilm must be generally thick enough to give a total thickness of about56 nm in order to provide an optical density of 3.0, suggests that thephotomask blank of the invention allows for film thickness reductioneven when a light-shielding film and an antireflective film arelaminated.

Dependency of Reflectance on Thickness and Wavelength

For the photomask blanks of Examples 6-10, the wavelength dependency ofreflectance was determined by a spectrophotometer, provided that lightwas incident on the film surface side. The results are shown in Table 5and FIG. 6. As seen from Table 5 and FIG. 6, a reflectance of 10 to 20%is obtained at the wavelength 257 nm or 365 nm when the antireflectivefilm has a thickness in the range of 19 to 37 nm, proving a possibilityof inspection with a commercially available defect detector.

TABLE 5 Reflectance (%) OD Wavelength 257 nm Wavelength 365 nm Example 62.78 15.6 33.7 Example 7 3.00 12.5 27.4 Example 8 3.22 7.7 18.7 Example9 3.66 6.5 10.8 Example 10 3.72 17.7 25.2 Example 11 3.94 15.5 19.0Example 12 4.39 16.0 14.2

Chemical Stability (Chemical Resistance)

The photomask blanks were immersed for one hour in an ammonia-hydrogenperoxide mixture (aqueous ammonia:hydrogen peroxide:water=1:1:30 involume ratio) or a sulfuric acid-hydrogen peroxide mixture (sulfuricacid:hydrogen peroxide=4:1 in volume ratio). A change of reflectance wasdetermined by a spectrophotometer UV-2400PC (Shimadzu Corp.). Under boththe conditions, the change of reflectance at wavelength 365 nm was lessthan 1%, demonstrating the elimination of any practical problems.

Dry Etching

On the photomask blank of Example 6, a resist pattern was formed by EBlithography using a chemically amplified resist (film thickness 180 nm).Using the resist pattern as an etch mask, CF₄ dry etching was carriedout (CF₄=80 sccm, 60 W, 2 Pa). A cross-section of the resultingstructure was observed under a scanning electron microscope. As aresult, the shape of the etched cross-section was satisfactory, and nosteps were found between the light-shielding film and the antireflectivefilm, proving that the light-shielding film and the antireflective filmcan be patterned in one step by fluorine dry etching.

Separately, on the photomask blank of Example 10, a resist pattern wasformed by EB lithography using a chemically amplified resist (filmthickness 100 nm). The CrON antireflective film was patterned bychlorine/oxygen dry etching (Cl₂=80 sccm, O₂=2 sccm, 60 W, 2 Pa). Across-section of the resulting structure was observed under a scanningelectron microscope. As a result, the shape of the etched cross-sectionwas satisfactory, demonstrating no significant invasion of thelight-shielding film by etching. Next, the resist was removed from theblank in which the CrON antireflective film had been patterned, andfluorine dry etching was then carried out (CF₄=80 sccm, 60 W, 2 Pa). Asa result, the shape of the etched cross-section was satisfactory, and nosteps were found between the light-shielding film and the antireflectivefilm. It is seen from these results that the CrON antireflective filmcan serve as a hard mask when the light-shielding film is patterned.

From the foregoing results, it is seen that the necessary thickness ofthe resist used in producing a photomask from the photomask blank of theinvention can be dramatically reduced.

Example 13

A halftone phase shift mask blank was fabricated and then processed intoa halftone phase shift mask as follows.

At the start, a first layer of 10 nm thick was deposited on a quartzsubstrate of 6 inch square by a sputtering deposition process,specifically by using a MoZrSi₄ sintered body and a Si single crystal assputtering targets, feeding a discharge power of 560 W and 1,000 W tothe MoZrSi₄ and Si targets, respectively, and rotating the substrate at30 rpm. During the process, a gas mixture of 8 sccm of Ar, 20 sccm ofN₂, and 5 sccm of O₂ was introduced as the sputtering gas. The gaspressure during sputtering was set at 0.15 Pa.

Then, a second layer of 40 nm thick as shown in Table 1 was depositedwhile changing the discharge powers so as to apply 430 W to the MoZrSi₄target and 1,000 W to the Si target, changing the sputtering gas to agas mixture of 15 sccm of Ar, 100 sccm of N₂, and 1 sccm of O₂, rotatingthe substrate at 30 rpm, and setting a gas pressure of 0.25 Pa.

Further, a third layer of 20 nm thick was deposited while changing thedischarge powers so as to apply 430 W to the MoZrSi₄ target and 1,000 Wto the Si target, changing the sputtering gas to a gas mixture of 5 sccmof Ar, 50 sccm of N₂, and 1 sccm of O₂, rotating the substrate at 30rpm, and setting a gas pressure of 0.1 Pa. A halftone phase shift filmwas obtained in this way.

Next, on the halftone phase shift film, a light-shielding film ofmolybdenum silicide and an antireflective film of molybdenum silicidenitride as in Example 6 were overcoated by the same procedure as inExample 6 to a thickness of 10 nm for the light-shielding film and 20 nmfor the antireflective film. This yielded a halftone phase shift maskblank having a light-shielding film and an antireflective film laminatedthereon.

On the halftone phase shift mask blank, a resist pattern was formed byEB lithography using a chemically amplified resist (film thickness 250nm). Using the resist pattern as an etch mask, CF₄ dry etching wascarried out (CF₄=80 sccm, 60 W, 2 Pa) for etching the antireflectivefilm, light-shielding film and halftone phase shift film. The endpointof etching was detected from a reflectance change while monitoring areflectance.

Next, to etch the light-shielding film on the halftone phase shiftpattern, the resist pattern was first stripped by a standard technique,and a negative resist was coated to form a resist film again. Patternirradiation was carried out to an outer frame portion where alight-shielding pattern was to be left, after which light was irradiatedto the entire surface of the substrate being processed from the backsidein order to protect the portion of the substrate surface where thehalftone pattern had already been etched away. The resist film wasdeveloped, forming a resist pattern that the resist was left only in theouter frame portion and the portion where the halftone pattern wasabsent. Using the resist pattern as an etch mask, CF₄ dry etching wascarried out (CF₄=80 sccm, 60 W, 2 Pa). At the point of time when theantireflective film had been etched, the light-shielding film was etchedby Cl₂ dry etching (Cl₂=80 sccm, 60 W, 2 Pa). For each of the films, theendpoint of etching was detected from a reflectance change whilemonitoring a reflectance.

A cross section of the mask pattern was observed under a scanningelectron microscope, finding a satisfactory etched shape. The halftonephase shift film was also found acceptable in phase and transmittance.

Example 14

By the same procedure as in Example 13, a halftone phase shift film wasdeposited on a quartz substrate. On this halftone phase shift film, alight-shielding film of molybdenum silicide and an antireflective filmof chromium oxynitride as in Example 10 were overcoated by the sameprocedure as in Example 10 to a thickness of 10 nm for thelight-shielding film and 20 nm for the antireflective film. This yieldeda halftone phase shift mask blank having a light-shielding film and anantireflective film laminated thereon.

On the halftone phase shift mask blank, a resist pattern was formed byEB lithography using a chemically amplified resist (film thickness 100nm). Using the resist pattern as an etch mask, chlorine/oxygen dryetching was carried out (Cl₂=80 sccm, O₂=2 sccm, 60 W, 2 Pa) forpatterning the CrON antireflective film. Then CF₄ dry etching wascarried out (CF₄=80 sccm, 60 W, 2 Pa) for etching the light-shieldingfilm and halftone phase shift film.

Next, the resist pattern was stripped by a standard technique, and aresist film was formed again. A resist pattern was formed so that theresist was left only where a light-shielding pattern was to be left.Using this resist pattern as an etch mask, chlorine/oxygen dry etchingas above was carried out to etch the antireflective film, after whichthe light-shielding film was etched by Cl₂ dry etching (Cl₂=80 sccm, 60W, 2 Pa). For each of the films, the endpoint of etching was detectedfrom a reflectance change while monitoring a reflectance.

A cross section of the mask pattern was observed under a scanningelectron microscope, finding a satisfactory etched shape. The halftonephase shift film was also found acceptable in phase and transmittance.

1. A photomask blank, from which a photomask comprising a transparentsubstrate and a mask pattern formed thereon can 5 be produced,comprising: a transparent substrate; and a light-shielding film, whereinthe light-shielding film is formed on the transparent substrate withanother film (A) intervening therebetween, the light-shielding filmcomprises one or more layers, at least one layer (B) of the layerscontaining silicon and a transition metal as main components, and thesilicon and the transition metal are present at a silicon/metal molarratio of 4/1 to 15/1.
 2. The photomask blank of claim 1, wherein theother film (A) comprises a phase shift film.
 3. The photomask blank ofclaim 1, wherein the transition 20 metal is molybdenum.
 4. The photomaskblank of claim 1, wherein the at least one layer (B) of the layers ofwhich the light-shielding film is composed further contains at least oneelement selected from oxygen, nitrogen and carbon.
 5. The photomaskblank of claim 1, wherein said light-shielding film has a thickness of20 to 50 nm.
 6. The photomask blank of claim 1, wherein saidlight-shielding film is overlaid with an antireflective film, and theantireflective film contains as a main component a transition metalsilicide oxide, transition metal silicidenitride, transition metalsilicide oxynitride, transition metal silicide oxycarbide, transitionmetal silicide carbonitride or transition metal silicide oxide nitridecarbide.
 7. The photomask blank of claim 6, wherein the transition metalsilicide is molybdenum silicide.
 8. The photomask blank of claim 1,wherein said light-shielding film is overlaid with an antireflectivefilm, and the antireflective film contains as a main component achromium oxide, chromium nitride, chromium oxynitride, chromiumoxycarbide, chromium carbonitride or chromium oxide nitride carbide. 9.A photomask comprising a transparent substrate and a mask pattern formedthereon, which is produced from the photomask blank of claim
 1. 10. Aphotomask blank in form of a phase shift mask blank, from which aphotomask comprising a transparent substrate and a mask pattern formedthereon can be produced, comprising: a transparent substrate; and alight-shielding film, an antireflective film and another film (A),wherein the light-shielding film and the antireflective film are formedon the transparent substrate with the other film (A) interveningtherebetween, the other film (A) comprises a phase shift film, thelight-shielding film comprises one or more layers, 30 at least one layer(B) of the layers containing silicon and a transition metal as maincomponents, the silicon and the transition metal are present at asilicon/metal molar ratio of 4/1 to 15/1, and combination of thelight-shielding film, the antireflective film and the phase shift filmhas an optical density of at least 2.5.
 11. The photomask blank of claim10, wherein the transition metal is molybdenum.
 12. The photomask blankof claim 10, wherein the at least one layer (5) of the layers of whichthe light-shielding film is composed further contains at least oneelement selected from oxygen, nitrogen and carbon.
 13. The photomaskblank of claim 10, wherein said light-shielding film has a thickness of20 to 50 nm.
 14. The photomask blank of claim 10, wherein saidlight-shielding film is overlaid with an antireflective film, and theantireflective film contains as a main component a transition metalsilicide oxide, transition metal silicide nitride, transition metalsilicide oxynitride, transition metal silicide oxycarbide, transitionmetal silicide carbonitride or transition metal silicide oxide nitridecarbide.
 15. The photomask blank of claim 14, wherein the transitionmetal silicide is molybdenum silicide.
 16. The photomask blank of claim10, wherein said light-shielding film is overlaid with an antireflectivefilm, and the antireflective film contains as a main component achromium oxide, chromium nitride, chromium oxynitride, chromiumoxycarbide, chromium carbonitride or chromium oxide nitride carbide. 17.A photomask comprising a transparent substrate and a mask pattern formedthereon, which is produced from the photomask blank of claim 10.